Characterizing the sources and atmospheric chemistry of light-absorbing carbonaceous aerosols using resonance Raman spectroscopy

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The proposed research will utilize ultraviolet resonance Raman (UVRR) spectroscopy to study the chemical composition of light-absorbing organic compounds in atmospheric aerosols. This family of compounds, known as brown carbon, is poorly understood even though research indicates that brown carbon has an important impact on radiative forcing of climate that varies widely depending on location. Moreover, carbonaceous aerosols in general - both organic and back carbon fractions - are currently the subject of intense scientific research because of their important impact on climate and human health. The goal of the proposed research program is to characterize the composition of brown carbon, identify the different sources of brown carbon, and understand the chemical processing of these light-absorbing organic compounds during their atmospheric lifetime.**UVRR spectroscopy is a unique tool for studying organic aerosol because it can provide vibrational spectra specifically for chromophores with an absorption band that matches the probing laser wavelength even when those chromophores are mixed with non-absorbing organics in a particle. The UVRR studies will investigate the products of organic reactions carried out in the laboratory that mimic the chemistry occurring in water-laden particles or cloud droplets, which have been linked to brown carbon production. The UVRR spectra of the `synthetic' brown carbon will be measured at multiple wavelengths to identify the chromophores responsible for light absorption. Different precursor compounds will be explored, and the resulting changes in brown carbon composition will be identified. In similar experiments, field samples of organic aerosol will be collected and artificially aged using ammonia gas, which has been show to produce light-absorbing nitrogen-containing organics. The spectra measured in the lab studies will be compared to those collected from field samples to evaluate the contribution of aqueous and cloud droplet chemistry to the observed brown carbon mass concentrations.**Analyses on field samples will be performed as well. In addition to quantifying the brown carbon functional group content, statistical methods such as positive matrix factorization (PMF) will be used to identify the different components of brown carbon. The correlations of the brown carbon spectral features and PMF components with emission tracers and photochemical age will then be utilized in the process of assigning different sources to the brown carbon components. The relationship between composition and light absorption will be explored using field data to find characteristics in the light absorption spectra that can be used to constrain brown carbon composition.**Many techniques have been applied to the study of organic aerosol composition including mass spectrometry, FTIR spectroscopy, and Raman Spectroscopy. These techniques provide valuable information, but none are capable of selectively analyzing brown carbon, or, at best, they characterize only a small fraction of the brown carbon mass. It is expected that the proposed research will provide new and more complete information regarding brown carbon sources and atmospheric chemistry. Ultimately, the results of this work can be used to constrain and test models of atmospheric aerosols, which are critical for predicting future climate.